Synergistic effects of different graphite on the braking performances of Cu-matrix friction materials for high-speed trains based on pin-disc tests

Abstract

Cu-matrix composites with the constant graphite content but different proportions were prepared by powder metallurgy technique for high-speed trains. A reduced proportion pin-disc testing machine was used to study the braking performances of friction materials at different speeds. The results show that the microhardness and resistance to plastic deformation of the friction surface are significantly affected by the proper introduction of colloidal graphite. Compared with copper-plated graphite, colloidal graphite is provided with more active surface free energy and can react readily with Fe to produce cementite. Cementite is a kind of metal compound with high strength, which has a great supporting effect on the material surface. Meanwhile, the cementite participates in the formation of the friction layer. As a result, the strength of the friction film is improved and the oxidation of Fe is reduced. All these are conducive to the generation of a friction film with strong anti-deformation ability. Then the transferred material of friction surface during high-energy braking is low, which stabilizes the coefficient of friction and decrease the wearing volume. However, too many colloidal graphite particles cause a lot of interfaces in the matrix and damage the matrix’s continuity greatly. With the rising colloidal graphite mass fraction, the copper-plated graphite’s arrangement direction changes. When colloidal graphite content is 1% and copper-plated graphite content is 19%, the friction material shows stable friction coefficient and has good tribological properties under high-speed braking of 350 km/h.